Not Applicable.
1. Field of the Invention
The present invention relates in general to the composition, preparation and application of a high quality adhesive composition for lignocellulosic, cellulosic and non-cellulosic materials. More particularly, the present invention relates to the composition, preparation and application of a saccharide-based adhesive resin for composite wood products, other lignocellulosic materials, and non-cellulosic materials. Even more particularly, the present invention relates to the composition, preparation and application of an adhesive resin containing corn syrup, water, isocyanate and polyol that is useful in the preparation of particle board and medium density fiberboard.
2. Background Art
Composite wood products prepared from various lignocellulosic materials (e.g., wood) have been known for centuries, although industrial production began only about a century ago. Among other things, these products offer a relatively low cost and a reasonably high strength to weight ratio. Consequently, they are used in a number of capacities including interior and exterior construction, furniture and even marine applications. Several distinct composite wood products exist including plywood, oriented strand board, particleboard and medium density fiberboard (MDF).
Products such as particleboard and MDF are typically prepared from an adhesive resin composition and comminuted lignocellulosic materials such as wood flakes or wood fibers. The manufacture of particleboard and MDF begins with the reduction of the wood particles to a particulate size or a fiber, which will occasionally be referred to herein as a furnish. This furnish is then blended with an adhesive resin and transported to a forming device, which shapes the mixture into a mat. The mat is then loaded into a heated press that shapes and pressurizes the mat to a desired thickness. The pressure and heat together act to cure the resin, which bonds the mixture into a panel or board. Bonding performance is affected by, among other factors, the amount and type of resin, the curing time and the curing pressure.
The predominant resin systems in the composite wood industry are urea-formaldehyde (UF) and phenol-formaldehyde (PF) resins. Although these formaldehyde-based resins perform consistently, the physical properties of composite wood products prepared with formaldehyde-based resins are often unsatisfactory. For example, the internal bond strength of composite wood products frequently renders them unfit for certain demanding applications. In addition, such products are commonly susceptible to significant swelling upon exposure to moisture. As a consequence of these and other factors, composite wood products prepared with UF and PF resins are often less robust than desired.
Currently, alternatives to formaldehyde-based resins are being investigated. The potentially attractive alternatives include resin systems that employ isocyanates. Such resin systems have been shown to impart markedly improved physical properties to composite wood products. For example, concentrated isocyanate-based resins increase bonding strength, and therefore offer a more robust and durable composite wood product. Unfortunately, isocyanate-based resins also have several known drawbacks that have limited their commercial utility. First, isocyanates are relatively expensive as compared to other resin materials. Consequently, concentrated isocyanate-based resins are uneconomical as compared with traditional formaldehyde-based resins. Second, unlike UF and PF resins, isocyanates are highly reactive with water. Consequently, isocyanates react quickly with any water present in either the wood furnish or the resin itself. This limits both the shelf-life of the resin and the lignocellulosic materials with which it can be used. Third, isocyanates are toxic and their use creates environmental, health and industrial hygiene concerns. Thus, process safety considerations influence against the use of concentrated isocyanate-based resins.
The chemistry of isocyanate binding of lignocellulosic materials is not well understood. However, it is well known that isocyanates react with the hydroxyl moiety on alcohols to give urethanes and that polyisocyanates react with polyalcohols (hereinafter 
xe2x80x9cpolyolsxe2x80x9d) to give polyurethanes. Because lignocellulosic materials have surface-terminated hydroxyl groups, it is commonly believed that isocyanates serve as an effective binder by forming urethane linkages with the lignocellulosic materials. It is also well known that isocyanate moieties react with water to yield less reactive ureas and carbon dioxide. Consequently, water is believed to damage the efficacy of the binder by reducing the availability of reactive isocyanate groups. 
The composite wood industry needs an improved resin capable of providing adequate physical properties while overcoming the limitations of the prior art. The improved resin should offer strong bonding, low cost, adequate shelf-life, and adequate process safety. Other objects and advantages of the invention will appear from the following description.
The preferred embodiments of the present invention comprise a novel resin for binding lignocellulosic, cellulosic and non-cellulosic materials into composite products. The components of the resin include one or more saccharides, polyisocyanates, and polyols in aqueous media. More particularly, some of the preferred embodiments of the present invention comprise a novel resin comprised primarily of saccharide with isocyanate, polyol and water as minor constituents. The preferred embodiments of the present invention also comprise a method for preparing the resin prior to its application to various lignocellulosic, cellulosic and non-cellulosic materials. The preferred embodiments of the present invention further comprise a method of applying the resin to various lignocellulosic, cellulosic and non-cellulosic materials to produce composite products such as particleboard and medium density fiberboard. In addition, the preferred embodiments of the present invention comprise the composite products prepared using the resin.